The invention relates to a method for controlling a clutch, in particular of a frictional clutch in a motor vehicle. Goal of this method is in particular to modulate the clutch torque that is transmitted during a starting process of the vehicle for making this starting process as comfortable as possible and to control it in a manner as desired by the driver.
In vehicles that are powered by a motor and have an automated transmission, it is common to use frictional clutches as starting components. Compared to for example torque converters in conventional automatic transmissions such clutches have the advantage that the clutch torques that are transmitted within the motor limits can be modulated almost entirely as desired.
In particular in the starting phase of a vehicle powered by a motor torque (a motor vehicle) high requirements apply to the control of a clutch for providing a starting phase that can be readily controlled, wherein a low speed low load range on the one hand like in case of parking and a high-power full load starting phase on the other hand have to be provided. The DE 197 168 28 introduces for this purpose various control methods that are described as generally possible controls. For example, it is disclosed that during the starting phase the transmitted clutch torque can be a function of the rotational speed of the motor.
In the DE 44 091 22 A1 it is suggested that the starting phase characteristic can depend on a pedal value since the starting phase rotational speed at the starting phase can be adjusted depending on that pedal value. Starting from this starting rotational speed the starting process is performed by controlling the slippage based on a target slippage characteristic. It is further disclosed that the control can also depend on the pedal position. However, the suggested pedal change is restricted to 30% since above this value the starting phase action is started anew or is terminated.
In the DE 102 306 11 A1 a method is disclosed where the starting phase characteristics differ from each other according to the chosen gear. In addition, the clutch torque that is transmitted by the clutch can be adjusted by a plurality of clutch characteristics, wherein one clutch characteristics sets the torque that can be transmitted at a particular motor load depending on the rotational speed of the motor. For this purpose, a standard clutch characteristic is provided that is then multiplied by a factor depending on a throttle valve angle, i.e. depends on the applied motor load. The factor is a linear function of the throttle flap angle that apart from constant values in the starting phase and the end phase decreases consistently. Therefore, the clutch torque decreases with larger throttle angle positions.
The DE 102 306 11 A1 suggests to introduce a factor that depends on time and influences the pre-existing characteristic that is determined by the standard clutch characteristic and the factor as described above. If during a starting phase action the throttle flap position changes, a change between clutch characteristics is conducted that differs depending on the factor that depends on throttle valve angle, wherein the signal from the throttle flap position is subjected to a strong filtering with long time constants.
The method according to the three documents as discussed above is only limited to starting phase characteristics, particularly in connection with supercharged diesel engines. Diesel engines often have an aggressive characteristic. This means, a strong torque increase takes place already at small gas pedal changes.
In addition, it is a desirable to achieve a starting phase with low pedal values and low rotational speed since higher rotational speeds result in undesired noise. Since also diesel engines develop their maximum torque at higher rotational speeds (>1600 rpm), for a full load starting action, the rotational speed of the motor has be controlled to a high value.